When installing oil and gas wells, and particularly underwater wells, it is frequently necessary to land a tool or a component on a support in an upright bore and to latch the landed device in place with the capability of retrieving the landed device. Since the location of the landed device is frequently inaccessible, latching must occur automatically, and if the device is to be retrieved, unlatching must be accomplished by operations carried out remotely, as with a retrieving tool manipulated by a handling string. Prior-art workers have employed for such purposes spring-urged latch members carried by, e.g., a hanger body and biased outwardly to engage automatically in a groove in which the hanger is to be landed. To provide for unlatching, the latch members have been provided with upwardly and inwardly slanting camming faces which, when the hanger or other device has been landed and latched, are exposed upwardly via an annular space, as seen for example in U.S. Pat. No. 3,171,674 issued Mar. 2, 1965, to Bickel et al. In most applications, the latch members are in the form of arcuate segments disposed in an annular series in an outwardly opening transverse annular groove. Since the latch members must be held in place on the hanger or other device, it has become common practice, as seen in FIG. 5 of U.S. Pat. No. 3,273,915 issued Sept. 20, 1966, to Bishop et al, to form the upper wall of the retaining groove with a dependent annular lip, against which the segments are urged by their springs, and to provide a gap or window in the lip so that the segments can be inserted through the window into the groove, and to provide a stop element to prevent the segments from realigning with the window. Such practices have achieved considerable success and commercial acceptance. However, forming the segment-retaining groove with a windowed lip is costly, and it has frequently proved to be unduly difficult to insert the segments properly. There has therefore been a continuing demand for improvement.